1. Field of the Invention
This invention relates to a composite polymer electrolyte membrane for polymer electrolyte membrane fuel cells. More particularly, this invention relates to a composite polymer electrolyte membrane which is able to operate at fuel cell operating temperatures of 60° C. and 150° C. and temperatures therebetween.
2. Description of Related Art
A fuel cell is an electrochemical device in which a fuel, such as hydrogen, is converted into electricity. The components of a fuel cell include an anode electrode, a cathode electrode and an electrolyte disposed therebetween. In a polymer electrolyte membrane fuel cell, the electrolyte is an ionically conductive membrane. For polymer electrolyte membrane fuel cells (PEMFC), the ionically conductive membrane should provide high ionic conductance with high strength, low electric conductance and chemical/electrochemical/thermal stability under fuel cell operating conditions. In conventional polymer electrolyte membrane fuel cells, the polymer electrolyte membrane is made of one or more fluorinated polymers, such as NAFION®, a fluorinated polymer membrane, and GORE-SELECT® membrane, a mixture of a per-fluorinated polymer and fluorinated ion exchange resin. However, the fluorinated polymer material is very expensive, as a result of which commercialization of the polymer electrolyte membrane fuel cell has been forestalled.
The requirements for a polymer electrolyte membrane in a polymer electrolyte membrane fuel cell include: (1) high proton conductivity at low relative humidity; (2) long life, that is long-term chemical/electrochemical/thermal stability, (3) low cost, and (4) low gas permeability. In order to meet these requirements, a stable polymer is necessary as a backbone; for example, NAFION uses a TEFLON® backbone structure.
Many proton conductive membranes are known in the art. These membranes are classified as follows:
1. Inorganic Membranes
The inorganic membranes known to be used in conventional polymer electrolyte membrane fuel cells are heteropolyacid, such as phosphotungstic acid (H3PO4.12WO3.xH2O), phosphomolybdic acid (H3PO4.12M0O3.xH2O), and silicotungtic acid (SiO2.12WO3.xH2O) and sol-gels, such as silica, titania, alumina, and zeolite. However, problems with the inorganic membranes in polymer electrolyte membrane fuel cells include very low conductivity and brittleness of the membrane. To date, to the best of our knowledge, no polymer electrolyte membrane fuel cell performance data for polymer electrolyte membrane fuel cells employing inorganic membranes have been reported.
2. Organic Polymer Membranes
Organic polymer membranes at present dominate polymer electrolyte membrane fuel cell development. At low operating temperatures, NAFION (DuPont), GORE-SELECT reinforced membranes (W. L. Gore), ACIPLEX® (Asahi Chemicals), FLEMION® (Asahi Glass), and BAM® (Ballard Power) are used. These membranes are limited to cell operation temperatures less than 100° C. with well humidified gas feeds and their manufactured costs are high. In contrast thereto, high temperature membranes focus on the use of sulfonated/phosphonated polymers, such as H3PO4 doped polybenzimidazole (PBI) or polyoxadiazoles. However, the stability of these high temperature membranes is low and the cost of the membrane is also high.
3. Composite Membrane
The composite membranes include inorganic-organic composite membranes and organic-organic membranes. The inorganic-organic membranes contain organic binders with inorganic acids, such as Zr-Phosphate+PTFE and Silicophosphate Gel Glass Composite; Sol-Gel Silane+PEO+HClO4; Silicophosphate Gel Composite+Porous Alumina Support+HClO4 etc. The organic-organic polymer membrane contains two or more organic polymers, such as PPSU (Polyphenyl sulfone)+PBO (Polybisbenzoxazole-1,4-phenylene) etc. However, to the best of our knowledge, no promising performance and lifetime data of polymer electrolyte membrane fuel cells with these membranes is known.